The use of flip-chip technology in the semiconductor industry is known. Specifically, with flip-chip technology, a series of bumps are formed upon a silicon substrate in order to facilitate physical and electrical connection of the die to a separate substrate. For example, flip-chip die have been attached to package substrates as well as printed circuit board substrates where device size is of utmost consideration. Once a flip-chip die has been attached to the corresponding substrate, it is necessary to dispense an underfill material into the space between the silicon die and the substrate. Generally, the underfill material is a silica-containing epoxy resin.
The formation of bump structures by evaporation onto the die requires the use of polyimide in order to facilitate proper attachment of the subsequent under bump metallurgy to the bump structure itself and to provide stress relief between the bump and the substrate. For example, it is common for a polyimide layer to be formed across the entire semiconductor die following the formation of the chip passivation layer. Openings through the polyimide to the underlying conductive contacts of the chip are formed to permit the desired contact of the bump to the substrate metallization.
The use of flip chip solder bump technology generally requires a reflow step after the die has been "bumped" with solder in order to form the final spherical bump structure on the die. The controlled collapse chip connection (C-4) bump technology has such an associated high temperature reflow step. With the exposure of the wafer to the high temperature reflow step, the above referenced polyimide material must be formulated to possess a high glass transition temperature (Tg) property above the maximum temperature reached during the reflow process. One problem resulting if the glass transition temperature is reached is that the polyimide layer on the wafer will expand in such a manner that the underlying and adjacent structures to the polyimide are physically damaged. In addition, even if damage were not to occur due to expansion of the polyimide, it is possible for the polyimide to return to a solid state having characteristics different than the original desired characteristics possessed prior to the reflow step. Therefore, physical damage due to expansion and a change of characteristics of the polyimide can cause immediate and long term reliability issues. For example, a change in the polyimide characteristics may limit the adhesion of the polyimide to underlying passivation layers, as well as to the adjacent Under Bump Metalilurgy (UBM).
In order for the glass transition temperature of the polyimide currently used for solder bump evaporation to remain sufficiently high, adhesion promoters are excluded from the polyimide formulation. However, with the removal of an adhesion promoter from the polyimide, it has been observed that where the flip-chip devices have been attached to an organic or ceramic board, that subsequent use or temperature cycling causes the adhesion between the underfill material and the polyimide region to deteriorate causing a failure to occur. The failure has been observed to be delamination at the interface of the underfill material and the polyimide material. Adhesion of underfill material to passivation is stronger than to polyimide possessing no adhesion promoters.
One effort in the prior art of dealing with delamination between a underfill material and a polyimide is to use adhesion promoters within the polyimide in order to change the Tg to a point where the properties of the polyimide are not affected. However, the use of adhesion promoters is not feasible where flip-chip solder bump technology is being used because of the required high-temperature reflowing of the bump structures. The problem being again that adhesion promoters actually lower the Tg of the polyimide. The lowering of the Tg causes the reflow step of flip-chip technology to affect the polyimide properties as described above. Therefore, a process capable of overcoming the problems of the prior art would be advantageous.